A conventional ultrasound imaging system comprises an array of ultrasonic transducer elements which are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. Such scanning comprises a series of measurements in which the steered ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received and stored. Typically, transmission and reception are steered in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
For ultrasound imaging, the array typically has a multiplicity of transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. Multiple firings may be used to acquire data representing the same anatomical information. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the time delay and amplitude of the applied voltages, the beam with its focal point can be moved in a plane to scan the object.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delays (and/or phase shifts) and gains to the signal from each receiving transducer element. The output signals of the beamformer channels are then coherently summed to form a respective pixel intensity value for each point of focus, corresponding to a sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as an image of the anatomy being scanned.
Tissue types and anatomical features are most easily differentiated in an ultrasound image when they differ in image brightness. Image brightness on conventional medical ultrasound imaging systems is a function of the receive beamformed signal amplitude, i.e., after coherent summation of the delayed receive signals on each transducer element. More precisely, the logarithm of the beamformed signal amplitude is displayed, with user-adjustable gain and contrast, and, if desired, a choice of a handful of grayscale mapping tables.
A human kidney usually appears in an ultrasound image as a darkish, ellipsoidal region (corresponding to the renal cortex) with a bright, irregularly shaped interior (the medulla). One criterion used by sonographers to evaluate ultrasound image quality is the contrast (i.e., the displayed difference in brightness) between the renal cortex and the medulla. This can be artificially increased by adjusting the grayscale maps manually after the fact, but this approach is of little practical value. Much more desirable would be the identification of another tissue contrast mechanism which could be used in addition to the receive amplitude to distinguish tissue types.